1. Field of the Invention
The disclosure relates generally to the field of device inspection by electron microscopy, and more particularly to handling samples made for such inspection.
2. Description of the Prior Art
Various devices, such as semiconductor products and disk drive transducers, comprise features that are fabricated at micron and smaller scales. For quality assurance and trouble-shooting purposes, it is often necessary to inspect certain features of such devices at different stages in the manufacturing process. Electron microscopy is a common tool for such inspections. Scanning electron microscopy (SEM) creates images by scanning an electron beam across a surface and measuring the electrons that are reflected towards an electron detector from features on the surface. Another technique, transmission electron microscopy (TEM), creates images by shining an electron beam on a very thin film and detecting those electrons that pass through the film. Features in the film interact with the electrons differently than the surrounding material, thus creating contrast in the resulting image. Although TEM typically provides greater spatial resolution than SEM, it will be appreciated that TEM requires highly specialized sample preparation to produce very thin films around the particular micron-scale and smaller features of interest. Such sample preparation is typically destructive to the device being inspected. The burden of sample preparation has kept TEM from being more widely accepted for routine inspections.
Existing techniques for TEM sample preparation in the field of device inspection employ focused ion beam (FIB) milling and are either in-situ or ex-situ. In-situ techniques produce the sample entirely within a FIB system, while ex-situ techniques complete the sample preparation outside of the FIB system. FIGS. 1–3 show the basic steps common to both sample preparation methods. FIG. 1 shows an initial device 100 having a feature of interest buried within at a reasonably well understood location.
In FIG. 2 a first pit 200 is milled into the device 100 on one side of the feature of interest. Next, a second pit 205 is milled into the device 100 on the other side of the feature of interest. Either of the first or second pits 200 or 205 is carefully enlarged in the direction of the other until only a very thin membrane 210 containing the feature of interest remains between the pits 200, 205.
As shown in FIG. 3, which shows the membrane 210 from the perspective of the section 3—3 of FIG. 2, a further thinned central portion 300 of the membrane 210 that includes the feature of interest can be created by additional FIB milling, leaving somewhat thicker side portions 305 for structural integrity on either side of the central portion 300. Next, the FIB system is used to cut the membrane 210 from the surrounding material, much like a welder's torch but on a much finer scale, to allow the membrane 210 to be free-standing. After a cut 310 is completed, the membrane 210 is held in place only by static electricity. The free-standing membrane 210 thus formed typically has dimensions of about 5 μm×15 μm. The central portion 300 is typically thinned to a thickness of about 100 nm or less, while the thickness of the side portions 305 is on the order of about 500 nm.
In the in-situ method, a very fine probe is first brought into contact with one of the side portions 305. Next, a film of platinum is deposited over the membrane 210 and the probe until the thickness of the platinum is sufficient to essentially weld the two together. The membrane 210 can then be lifted away from the device 100 and transferred to a specialized sample grid for TEM microscopy.
A disadvantage of the in-situ technique is that the membrane 210 must be cut from the probe by further milling. Additionally, the film of platinum must be cleaned from the surface of the membrane 210 by still further milling. Besides possibly disturbing the feature of interest through the welding, cutting, and cleaning processes, the in-situ method also ties up the FIB system, reducing throughput.
In the ex-situ method, a very fine probe is created by drawing a glass rod to form a sharp tip. The device 100 is removed from the FIB system and a mechanical translator is used to maneuver the probe into close proximity of one of the side portions 305 of the membrane 210. As the probe is brought close to the side portion 305, electrostatic forces between the glass of the probe and the membrane 210 cause the membrane to break free from the device 100, and in some instances causes the membrane 210 to jump to the probe. Unfortunately, in other instances the membrane 210 flies away from the probe.
Provided that the membrane 210 is successfully captured by the probe, the membrane 210 can then be maneuvered to a TEM support grid. The TEM support grid is a 3 mm diameter disk having a fine mesh of copper wires. A very thin film of holey carbon is deposited on the copper mesh. Ideally, the membrane 210 is placed on the holey carbon film such that the central portion 300 of the membrane 210 is aligned with one of the holes in the holey carbon. However, because the membrane 210 is held to the probe only by electrostatic forces, it is often difficult to position the membrane 210 precisely on the TEM support grid. While the ex-situ method is desirable over the in-situ method because it frees the FIB system for other work, and does not include the steps of welding the membrane 210 to the probe, cutting the membrane 210 from the probe, and cleaning excess platinum from the membrane 210, the ex-situ method suffers from low yield due to the unpredictability of the electrostatic forces on which it relies.
Accordingly, what is needed is a more reliable ex-situ method for producing TEM samples from devices.